My long-term interest is to elucidate molecular mechanisms governing mammalian development and selfrenewal and differentiation of stem cells. Here, I employ skin as a model system to investigate functions of a family of small RNA molecules, microRNAs, in embryonic development. Such research will provide future basis for clinical use of skin stem cells in regenerative medicine and for developing methods for prevention and treatment of skin cancer. In the K99 phase of this project, I showed the differential expression pattern of miR-203 during embryonic skin development and demonstrated its critical role in repressing the prolifeartive potential of epidermal stem cells once these cells enter the terminal differentiation program. Furthermore, I identified p63, an essential gene for the maintenance of epidermal stem cells, as a key target of miR-203. These studies provide the first example of an individual microRNA that plays an important role in skin. In the ROO phase, I will pursue the initial studies and probe more deeply into this hitherto unexplored area of skin biology. Specifically, I will first utilize accurate isolation of specific skin lineages by fluorescen-actived cell sorting method and high-throughput microan-ay profiling of microRNA expression to identity differentially expressed microRNAs in specific epidermal and hair lineages. I will then use in situ hybridization method to monitor the expression patterns of these microRNAs during skin development. This study will reveal diffemtial expression of microRNAs in high resolution and provide candidates for future functional studies. To further explore the mechanism of miR-203's functions, I will establish an in vivo mouse model to identify the direct target of miR-203. This study will elucidate the molecular mechanisms of miR-203's functions and improve our understanding of microRNA target recognition. Collectively, these studies will also pave the way for me to establish my independent research program and prepare preliminar data for a R01 application.